1. Field of the Invention
This invention relates generally to electrical protector devices used in various locations such as telephone central offices to protect the inside equipment from damage as a result of over-voltage and over-current conditions that may occur on the incoming lines and, more particularly, to a protector device which uses a heat coil to provide protection against sneak currents.
2. Description of the Prior Art
In modem telephone engineering, it is usual practice to provide protectors at central offices for each incoming telephone line. These protectors provide protection against excessive voltages resulting, for example, from lightning. In addition, the devices provide protection against over-currents and so-called sneak currents which are not strong enough to do any damage if they flow briefly but which may generate enough heat to char insulation and do other damage if allowed to persist. The sneak currents are produced by voltages of relatively low magnitudes as compared to the excessive voltages first mentioned and usually result from accidental interference between telephone lines and adjacent power lines.
There are many well-known devices which are used at telephone company central offices and increasingly at other locations to provide protection against any over-voltage and over-current conditions that may occur on a telephone line pair. One such device uses a gas tube to provide protection against an over-voltage condition on the telephone line pair to which the device is connected. In operation, the gas-filled tube ionizes at a predetermined voltage level to conduct surging currents to ground potential. These tubes are normally employed in conjunction with heat sensitive elements which become operative on the occurrence of sustained excess current loads, as distinguished from momentary current surges.
One type of heat sensitive device used to protect against over-currents and sneak currents is a wire wound heat coil which is supported upon a bobbin and supports an accompanying coil spring actuator to effect a grounding action. One example of such a device is that shown in U.S. Pat. No. 4,215,381 (hereinafter the '381 patent). The heat coil described in the '381 patent, for sensing excessive or sneak currents, includes a spool having a first conductive flange on one end, a winding of resistance wire about an outer surface, and a pin affixed to an inner surface by a thin coating of solder. A second conductive flange is spaced apart and isolated from the first flange by an insulative member. One end of the resistance wire windings is connected to the spool and an opposite end of the windings is connected to the second flange. Also included is apparatus, surrounding the heat coil and in contact with the second flange, for urging the heat coil subassembly first flange from a first position spaced apart from the ground plate subassembly to a second position in contact with the ground plate subassembly upon passage of excessive currents or sneak currents through the resistance wire windings.
The input impedance of the heat coil used to protect telecommunication lines and equipment connected thereto must satisfy certain specifications. Specifically, the input resistance of the heat coil should have small total resistance. Unfortunately, the heat coils presently in use for sensing excessive currents or sneak currents have a relatively high input resistance.
Another device such as a Positive Temperature Coefficient (PTC) resistor can be used in place of a heat coil for protection against excessive currents and sneak currents. Unfortunately, a PTC does not switch the excessive current or sneak current to ground. A low-impedance PTC will not be able to meet all the safety requirements, whereas the high-impedance needed for safety can degrade the signal with too much loss.